A continuous bound placket is one of my favourite methods for a sleeve placket where lightweight fabrics are used, such as on a blouse or dress. It is easily inserted into a one piece sleeve where a vertical split is made – not at the sleeve seam, but at the location of the little finger.
You will need to cut the binding twice the length of the split, and a cut width of 25mm (1″) will give a finished width of 6mm (1/4″). The binding is easier to sew if it is cut on grain, not the bias. If you are sewing checks or stripes, then cutting on the bias is a good idea to avoid pattern matching.
Here I have opened out the sleeve split and laid it over top of the binding, with the right side of the binding to the wrong side of the sleeve. The raw edges align at either end of the split, but at the apex of the split they are offset by 5mm – this is how it will be sewn:
If you are finding this a bit tricky – sew the first few stitches and stop with your needle down, then you can position your fabric again. Making the binding taut by pulling it slightly towards you helps to form an even binding width. Pull both top and bottom layers evenly so that the binding does not twist like a rope.
It is best not to press the edge down first, with a bit of practise you will actually achieve a neater result if it is pressed after you have edgestitched.